There has been a method of using the rotation speeds of wheels attached to a vehicle to indirectly detect whether a tire has a decreased pneumatic pressure or not (see Japanese Unexamined Patent Publication No. 2005-1419 and Japanese Unexamined Patent Publication No. 2006-1298 for example). Conventionally, the practical realization of the detection method as described above has required a process called a calibration process in which an actual vehicle test is carried out to examine the logic used for such a detection method and to optimize parameters used in the method. Specifically, a development vehicle in which a deflation warning apparatus is to be installed was obtained. Then, an actual vehicle test was carried out to run the development vehicle on a test course or the like. Through this test, there was obtained information from various kinds of sensors such as the wheel rotational speeds of the respective four wheels, a lateral G, a yaw rate, a steering angle, and a wheel torque (information required to detect a decreased pneumatic pressure). Then, the obtained test data was analyzed to thereby perform the calibration process of each vehicle. In particular, an alarm threshold value for determining whether a decreased pressure alarm is issued to a driver or not is obtained by carrying out a straight running at a fixed speed based on a plurality of speed standards to thereby calculate the threshold values based on the obtained test data.
However, since the actual vehicle test requires the development vehicle itself, the completion of the development vehicle must be waited and limitations are also caused such as the limited number of test vehicles and the limited period during which the test can be carried out. Furthermore, the test is actually carried out by a test driver. This consequently requires a large amount of costs such as a personnel cost, a vehicle maintenance cost, and a fuel cost and also causes a disadvantage in which various enormous manhours such as the ones for measurement preparation and data analysis, and a long-term development period are required. Furthermore, when a minor change is performed to a development vehicle already subjected to a calibration process, a new actual vehicle test must be performed to this development vehicle subjected to the minor change. This has required a large amount of cost and a long time for the completion of the calibration process of one type of development vehicle.